1. Field of the Invention
This invention relates generally to systems and methods for detecting scratches on surfaces of wafers, and more particularly to systems and methods for detecting scratches on surfaces of wafers made of non-semiconductor materials.
2. Description of the Related Art
With the reduction of design feature size and elevation of device reliability standards, the device manufacturer has imposed an increasing demand for better substrate surface quality, calling especially for the elimination of scratch defects. The device manufacturer is now requiring that substrates be free of polished surface scratches at the device application level. This requirement is well beyond our current inspection capability for scratch defect detection. To meet this quality control requirement, it is imperative for substrate manufacturers to possess/develop the ability to detect scratch defects in a more effective and efficient manner while also accommodating production output increases.
The objective of the invention is to develop an inspection system with much higher sensitivity in detecting scratch defects than what is visible when using the high-intensity light, un-aided eye visual inspection technique. Additionally, the invention will significantly improve upon the throughput limitations experienced when using commercial laser inspection systems, such as the Candela CS2 product.
Currently, visual and laser scanning are two methods used when inspecting for polished surface scratches. Neither of these methods is effective in capturing subtle scratch defects.
With visual inspection, a skillful technician holds a wafer under illuminating light in a dark room and/or surrounding to determine if a scratch is present on the wafer surface. This is achieved based on scattering from the scratch as the wafer is tilted at an angle of incidence to the light. However, the visual inspection method has three main drawbacks,
(i) Poor capture rate: This is due to the transmission nature of lithium niobate wafer under visual light. Scattering from backsides' rough surface confounds the scatter from the scratch on the front surface, thereby greatly reducing the sensitivity of scattering from a scratch;
(ii) Inconsistency: The inspection ability is highly skill-dependent;
(iii) Labor intensive: It is direct labor involved for each wafer inspection.
To achieve a high capture rate, a laser inspection system based on low incidence angle is generally deployed. It does offer the ability to capture the scratch under submicron. However, due to the smaller laser beam size (5˜50 um), the inspection throughput is too low to meet the production volume demands. High operation cost together with a steep initial capital investment commitment has prohibited the system from being used 100% for scratch screening in production, especially in the case where the fabrication cost of substrates is very competitive.
The current scratch detection methods available are either not effective as required or are cost prohibitive. Accordingly, there is a need for an effective and efficient inspection method at a reasonable investment cost.